Edit the Custom BB Code [streetview] so the link located in the HTML Replacement is pointing to streetview.php is located in your /misc/ folder.

How to use:

Go to Google maps.

Select a location and click street view.

Copy URL in address bar.

Under the Visitors tab, click the Street View link.

Paste the URL into the form and click submit.

Copy the streetview code and paste it into a message

Error message:

If the add-on cannot convert a Google street view URL successfully, an error message "The URL entered cannot be converted" will be displayed. Make sure you're viewing a Google street view when copying a URL.